Hmm, I Don't have the DLC for Blue Dragon, BUT I can say for certain (120 hours gametime certain that is >.>) that you can't "upgrade" your accessories, they are what they are.
HOWEVER, you can get class skills with SP farming to unlock more accessory equip slots for special accessories such as "Lei" and "Black Belt" for EXP+50% and Double SP.
I hope this is of help :)
Awesome,much?
